1. Field of the Invention
This invention relates to a photoreceptor for electrophotography which comprises a photoconductive layer composed mainly of amorphous silicon.
2. Description of the Prior Art
In photoreceptors provided for electrophotography presently in practical use, a combination of high resistance and high sensitivity is a basic requirement. As a material having such a combination of characteristics, a resin dispersal material in which cadmium sulfide powder is dispersed into an organic resin and an amorphous material such as amorphous selenium (a-Se) or amorphous arsenious selenide (a-As.sub.2 Se.sub.3) have been most often used. However, all such materials cause pollution, so the development of a substitute material is desirable. In recent years, amorphous silicon has gained prominence as a different material for the above-mentioned photoreceptors.
In addition to its not causing pollution and its having high sensitivity, this substance is also extremely hard, and it is expected to be a superior material for use in photoreceptors. However, amorphous silicon by itself does not have enough resistance to maintain the electrostatic charge necessary during the procedures of electrophotography. Therefore, in order to use amorphous silicon as a photoreceptor for electrophotography, a means by which a large electrostatic potential can be maintained with high sensitivity is necessary.
As one such means, it has been proposed to bring about high resistance in the amorphous silicon layer itself which is to become a photoreceptor. However, in order to efficiently use the superior photoconductive characteristics of amorphous silicon (including its strong optical absorbance, relatively large drift mobility of electrons and positive holes, its sensitivity to long wavelengths, etc.), it would be better to provide a blocking layer having a great energy bandgap on the surface of each of the amorphous silicon layers (i.e., on the photoconductive layer) and the substrate rather than to enhance the capacity to be charged with electricity by bringing about high resistance in the photoconductive layer itself, as has been mentioned. This kind of surface layer with a great energy bandgap does not only hold an electrostatic charge, but also protects the photoreceptor from strong corona shock arising during the process of electrophotography. Such a surface layer also acts as a protective film which minimizes changes in the characteristics of the photoreceptor caused by changes in the environment (in temperature, humidity, etc.) so as to stabilize the surface of the photoreceptor; such a protective surface layer is indispensable. Of course, for it to act as a protective surface layer, a great energy bandgap is desirable for this layer.
As mentioned above, the provision of the surface layer having a great energy bandgap is desirable in that not only can an electrostatic charge be effectively held on the photoconductive layer, but also the surface of the photoconductive layer can be protected. However, when a surface layer with a great energy bandgap is formed directly on the amorphous silicon layer which is a photoconductive layer, various phenomena appear that are undesirable in a photoreceptor for electrophotography.
One such phenomenon is mechanical instability. When a photoconductive layer of amorphous silicon is constructed with a surface layer having a great energy gap, the binding between the photoconductive layer and the surface layer is not stable due to a difference in the coefficient of thermal expansion therebetween, and they tend to peel away from each other.
Another phenomenon is deterioration in the electrical characteristics of the photoreceptor. That is, during the process of electrophotography, when a photoreceptor, the surface layer of which has been already electrically charged, is illuminated, the light causes an electric charge on the photoreceptor with a different polarity from the charging polarity of the electric charge on the surface layer. The electric charge on the photoconductive layer then moves through the surface layer to neutralize electrostatically the electric charge on the surface layer. However, the energy bandgap of the surface layer is so large that there is an extremely great energy gap at the interface between the photoconductive layer and the surface layer, and smooth transfer of the electric charge does not take place. Instead, the electric charge builds up in the vicinity of the interface between the surface layer and the photoconductive layer, resulting in a residual potential. Such a residual potential is undesirable, and if it increases, it can cause deterioration in the characteristics of the photoreceptor.
Moreover, residual potential frequently gives rise to movement of the accumulated carriers in the horizontal direction of the photoreceptor, which causes the problem known as image blurring.
As mentioned above, a surface layer with a great energy bandgap is essential because it holds the electric charge and protects the surface of the photoconductive layer, but it causes incidental problems both mechanically and electrically. This means that a satisfactory photoreceptor made of amorphous silicon has not yet been achieved.
Moreover, in order to prevent the injection of an electric charge from the substrate to the photoconductive layer, it is preferable to form a bottom layer with a great optical bandgap on the bottom of the photoconductive layer which faces the substrate, in the same manner as in the surface layer with a great optical bandgap.
However, because of the lack of mechanical matching, it is difficult to form a photoconductive layer, which does not incorporate any nitrogen (N) or carbon (C) directly, with a thickness of, for example, 8 .mu.m or more on the bottom layer.
If an amorphous silicon membrane which does not include any boron is used as the photoconductive layer, it is not suitable for use as a photoconductive layer when positively charged because of a number of difficulties: the resistance is small, the capacity to be charged with electricity cannot be large, and the transport capacity (mobility-carrier life time product) of positive holes is poor.